1. Field of the Invention
The present invention relates to an imaging apparatus including an imaging element that reads out an image signal based on an interlace system and an imaging method using such an imaging apparatus.
2. Description of the Related Art
In recent years, the spread of digital cameras is admirable, and their performance is evolving on a daily basis. Further, recent digital cameras have various functions, whereby a photographer can take high-quality pictures irrespective of his/her skill. An autofocus function is one of these functions. To avoid missing a photo opportunity, it is necessary to shorten a time required for detecting an in-focus position as much as possible when the autofocus function is used.
Autofocus systems can be roughly classified into an active system and a passive system.
The active system is a system that irradiates a subject with auxiliary light such as an infrared ray from a camera, detects reflected light of this auxiliary light from the subject to measure a distance from the camera to the subject, and drives a focus lens in accordance with this distance. This active system is used in some digital video cameras.
The passive system is a system that measures a distance based on the luminous flux from a subject that has passed through an imaging lens, and it is divided into a phase difference system and a contrast system. The phase difference system is a system that divides a pupil of the imaging lens into a pair of regions and detects a relative positional change of a pair of images formed by the luminous flux that passes through the divided pupil regions to detect an in-focus position, and this system is often used in digital single-lens reflex cameras. The contrast system (which will be referred to as contrast AF hereinafter) is a system that is often used in compact digital cameras, and reads out an image signal from an imaging element while moving a position of a focus lens along an optical axis direction, calculates an AF evaluation value used for evaluating contrast from the image signal obtained in accordance with each frame, detects a maximum (peak) value of the AF evaluation values, and determines a focus lens position where the peak value of the AF evaluation values can be obtained as an in-focus position.
Here, as one of methods for detecting an in-focus position at a high speed in the contrast AF, increasing a frame rate for reading out an image signal from the imaging element can be considered. As one of systems for increasing a frame rate for reading, there is an interlace system. The interlace system is a system that divides one frame into a plurality of (e.g., two which are an Odd (odd number) field and an Even (even number) field) fields and reads image signals from different pixels on the imaging element in the respective fields. According to such an interlace system, the frame rate for reading from the imaging element can be increased.
When the interlace system is adopted as a system for reading an image signal from an imaging element, a frame rate for reading can be increased, but an AF accuracy is reduced. This reduction in AF accuracy will now be described with reference to FIG. 6. FIG. 6 shows an example of two-field reading, and C1 and C3 represent AF evaluation values of an Odd (odd number) field. Furthermore, C2 and C4 represent AF evaluation values of an Even (even number) field. In the case of reading an image signal based on the interlace system, AF evaluation value calculation areas do not spatially coincide with each other in the Odd field and the Even field. Therefore, even if focus lens positions are equal to each other, an AF evaluation value calculated in the Odd field may be possibly different from an AF evaluation field calculated in the Even field depending on a photographic scene.
As described above, in the contrast AF, a peak value of the AF valuation values is detected. Here, when a drive speed of a focus lens increases, an interval for acquiring each AF evaluation value widens, and a possibility that an AF evaluation value at a true peak position cannot be obtained rises. Therefore, to detect the true peak position, an interpolation calculation is required. For example, in FIG. 6, C3 is determined as a temporary peak position of the AF evaluation values. In the interpolation calculation, an interpolation curve running through three AF evaluation values C2, C3, and C4 at three points including this temporary peak position C3 is calculated, and a maximal value of this interpolation curve is obtained, whereby a peak position is calculated.
Here, as described above, since how each AF evaluation value differs in the Odd field may highly possibly differ from that in the Even field, a peak position different from the true peak position may be calculated by the interpolation calculation. When a wrong peak position is calculated, since a position deviating from the true in-focus position is determined as the in-focus position and a focus lens is driven, an AF accuracy is lowered.
In regard to a reduction in AF accuracy in the contrast AF when such an interlace system is used, according to Jpn. Pat. Appln. KOKAI Publication No. Hei 1-268366, a high-pass component in an image signal obtained from an imaging element is acquired as a first evaluation value in accordance with each field, an evaluation value obtained by adding the acquired first evaluation values every two continuous fields is determined as a second evaluation value, and a position of a focus lens is adjusted so that this second evaluation value can be maximum (maximal).
Further, as a suggestion similar to Jpn. Pat. Appln. KOKAI Publication No. Hei 1-268366, in Jpn. Pat. Appln. KOKAI Publication No. 2003-140032, image signals in fields are acquired at respective focus lens positions, the image signals of the fields are added up, an AF evaluation value is calculated from the added image signal, and the contrast AF is carried out. In Jpn. Pat. Appln. KOKAI Publication No. 2003-140032, an in-focus position can be detected by using the added image signal even if a subject has low luminance.